In this regard, WINS is a known term. WINS stands for Wireless Indoor Solutions offering to customers RF communication systems for confined areas. It enables users to use their mobile radios and cellular phones in areas normally not covered with RF signals. The aim of such communication systems is to distribute cellular services using a cost-efficient and broadband infrastructure, wherein the desired cellular services can be distributed over different frequency bands (e.g. GSM900, GSM1800, UMTS2100, SMR800, CDMA800, PCS1900, WiMax2500, etc.).
However, the need for a service can change over time, and one service can be switched-off while another one has to be switched-on. Also technology evolution (e.g. from GSM900 via UMTS to WiMax) makes an easy upgrade of the services in such communication system necessary. In general, said systems are exposed to the following challenges (e.g. inside buildings): distribution of multi-band cellular services; distribution of multi-operator cellular services; cost-efficient broadband infrastructure that allows for distribution of cellular services independent of the frequency allocation; enabling an easy upgradeability to other frequency bands or combinations and an extension of the coverage area; distribution of frequency bands that directly border or that have overlapping frequency bands (especially for the FCC (Federal Communications Commission) services in the US).
State-of-the-art multi-service communication systems make use of several large-scale and expensive solutions to solve the technical problems mentioned above. The most common way is the use of high-end filters and duplexers to provide sufficient isolation between the services. Further solutions propose the installation of a parallel infrastructure for each service. Still further solutions propose a frequency conversion technology in order to save the effort of filtering in the RF domain resulting in several technical problems e.g. frequency accuracy and stability. Basically, conventional solutions can be divided into three main categories, namely passive systems, active systems, and hybrid systems. In the following, the principle of such systems is outlined.
In known passive systems signals from a centralized source, e.g. Base Transceiver Station (BTS) or Node B or off-air repeater, are combined via a point of interconnection (POI) and fed directly to a coaxial distribution system. The coaxial distribution system can consist of coaxial cables, couplers and RF splitters. Such passive distribution system feeds a distributed antenna system (DAS) consisting of antennas and/or radiating cables. However, the following problems arise from the known passive systems. A coverage area is restricted depending on the output power level of the active equipment and of the high passive losses of the passive distribution network and DAS. That is, the losses of the distribution infrastructure have to be compensated by high output power level of the active equipment, for example the signal source. Furthermore, the extension of such systems to larger coverage areas is complicated, because a second BTS/Node B location has to be installed in confined areas (e.g. in a building) as well as a second independent distribution topology. In addition, a sectorization and capacity optimization has to be chosen carefully during system design. For example, it is not possible to change the sectorization inside a building after the installation. A further problem of known passive systems is that depending on the frequency allocation of the cellular services a high effort for RF filtering is required in case of bordering services. A distribution of overlapping services is only possible if individual infrastructures for uplink (UL) and downlink (DL) bands or a combination thereof is used. That is, in such case two separate networks are used to transmit suitable combinations of UL and DL RF bands.
In known active systems signals from a signal source, e.g. BTS or Node B or off-air repeater, are combined via a point of interconnection and distributed in a RF network comprising cables and antennas, similar to the above-described passive systems. The RF losses are compensated mostly in bi-directional amplifiers or amplifier cascades, for example depending on the network size. In complicated cases of frequency allocations, individual networks for UL and DL are used or combinations of both. However, the following problems arise from the known active systems. The RF losses of the distribution infrastructure have to be compensated by a high output power level of the active equipment, e.g. bi-directional amplifiers. For several services such bidirectional amplifiers must be combined with additional (band-selective) filter elements. However, such solution results in high costs and therefore the number of bi-directional amplifiers must be optimized and the need of high-end RF infrastructure with low RF losses is the consequence. In addition, in most of the known active systems intermodulations are generated which are difficult to avoid.
Also in known hybrid systems signals from a signal source, e.g. BTS or Node B or off-air repeater, are combined via a point of interconnection and fed to a master unit, which includes a conversion of the analogue RF signal into an optical signal in DL. By the use of optical fibers and specific electro-optical units high RF losses in larger RF networks are avoided and large distances can be overcome. The optical signal is distributed via the optical fibers in the confined area. A remote unit re-converts the optical signal into the original RF signal, which is amplified afterwards. The amplified RF signal is distributed via passive DAS. For UL the system works vice versa, including RF to optical conversion in the remote unit and optical to RF conversion in the master unit. However, the following problems arise from hybrid systems. Due to fiber-optical signal distribution and due to expensive installation costs for optical fibers a high installation effort is required. Compared to passive systems the same effort for RF filtering has to be implemented in the remote units in order to isolate the uplink and the downlink of the systems. Furthermore, the design of multi-service systems is often only possibly by parallelization of each service in order to avoid interferences. Thus, the costs for purchasing and installation of multi-service hybrid systems are very high. Furthermore, optical systems produce high system noise reducing the system dynamic range. In addition, it is difficult to optimize the system dynamic range between limitations of noise and intermodulations.
Consequently, all analogue system solutions need a careful signal leveling to provide sufficient radiated power for requested coverage and to avoid eventual intermodulations due to possible over-drive of active units. In addition, network extensions are mostly complicated due to re-configurations and new signal leveling.
It is therefore an object of the present invention to provide an improved signal transmission in a radio frequency network which avoids intermodulations and which reduces the effort for network commissioning and testing when installing the network for the first time and when extending the network.
This object and other objects are solved by the features of the independent claims. Preferred embodiments of the invention are described by the features of the dependent claims.